1. Field of the Invention
The present invention relates to a Coriolis mass flow meter and, more specifically, relates to a small-sized mass flow meter applicable to measurement of a minute flow rate.
2. Description of the Background Art
Since a Coriolis mass flow meter has an advantage of being capable of directly measuring a mass flow rate with accuracy, historically, its application started to measure a large flow rate, and has recently been expanded to measure a minute flow rate.
A principle of the Coriolis mass flow meter is as follows. When a central portion of a vibration tube in an axial-line direction whose a fluid flows is excited, Coriolis forces act in opposite directions in an inlet side portion and an outlet side portion of the vibration tube, and the Coriolis forces in the opposite directions cause generation of twist in the vibration tube. This twist is proportional to the mass flow rate. This twist of the vibration tube is detected as a vibration displacement, a velocity phase difference, or the like between the upstream side and the downstream side in a flowing direction of a fluid with the vibration exciter interposed therebetween, to obtain a mass flow rate from this phase difference or the like.
There have already been proposed vibration tubes having a variety of shapes in Coriolis mass flow meters. The vibration tubes can be broadly divided into a straight tube type which is straight and a curved tube type provided with a curved portion. The curved tube type is typically classified into a type of the inlet and the outlet of the vibration tube being located on the same side and a type of the inlet and the outlet being located on the opposite sides. As for the former type, namely the type of the inlet and the outlet being located on the same side, those having a U-shape in a planar view, a shape of a narrowed space between the inlet and the outlet (Japanese Unexamined Patent Publication No. 2003-207380), a loop shape, and the like are known. Further, the Coriolis mass flow meters are classified, based upon the number of vibration tubes, into a single tube type and a dual tube type (Japanese Unexamined Patent Publication No. 2003-207380 and U.S. Pat. No. 4,756,198) provided with two vibration tubes.
Japanese Unexamined Patent Publication No. 2003-207380 discloses a dual tube type Coriolis mass flow meter, to which two curved tube type vibration tubes that vibrate as a tuning fork, namely a pair of vibration tube, are adopted for the purpose of application to a fluid with a minute flow rate. It is pointed out as reasons for adopting the two curved tube type vibration tubes that a single tube type has a low drive efficiency and is unbalanced when vibrated, thereby causing the problem of occurrence of vibration leakage, and that the single tube type requires a frame structure with enhanced rigidity to support the detector, namely rigidity against vibrations.
Here, the vibration leakage is described. With a pair of vibration tubes ideally vibrating in a mirror-symmetrical manner, a vibration due to a self-excited vibration is offset on the frame. However, this is an ideal theory, and the tubes are in effect not completely mirror-symmetrical because of their material properties, shapes, assemblies, or the like being nonhomogeneous, nonuniform, or asymmetrical. Hence, the vibrating states in minute vibrations of the vibration tubes associated with the frame or an external pipe vary depending upon the assembled states thereof. This leads to an offset of a zero point of a measured value. This phenomenon is referred to as the vibration leakage. This vibration leakage occurs independently from a vibration from the outside as a disturbance element.
A vibration tube using a metal material (typically, stainless steel) has been traditionally adopted to the Coriolis mass flow meter. Japanese Unexamined Utility Model Publication No. S64-15921 (Japanese Utility Model Application No. S62-107307) proposes a vibration tube formed by arranging a tube of a synthetic resin, such as a fluorine resin as a material having excellent corrosion proof properties, on the inner periphery of a metal tube in order to pave the way for application of the Coriolis mass flow meter to medical agents which are acid, alkaline and the like.
Further, Japanese Translation of PCT Publication No. H11-510608 discloses constituting a vibration tube of a plastic material having excellent corrosion proof properties, and cites as examples thereof polytetrafluoroethylene (PAFE), a tetrafluoroalkoxy polymer (PFA), and polyether ether ketone (PEEK).